This invention relates generally to the art of magnetic coded data cards and the like and more specifically relates to improved magnetic encoding devices and the method for making the same.
It has long been known that discontinuities are created in magnetic hysteresis loops when magnetic materials are placed under the influence of any stress. Stressing may take the form of tension, torsion, or bending. These are known as Barkhausen discontinuities. This known property of magnetic materials has been utilized in the art to prepare magnetic encoding elements for use in coded identification cards, key cards, security control cards, etc. The material generally used for this application is a magnetic alloy called Vicalloy, a composition of vanadium, iron and cobalt. This alloy is a commercially available material which may be easily worked, drawn into wire form or rolled into strip form. This material has been used in magnetic encoding devices in wire form which is strained by repeated twisting of the wire while applying a longitudinal tensile force. This method creates different magnetic conditions in the shell of the wire as contrasted to the core. The magnetic properties along the wire may be varied, if desired, by radial compression, for example. Magnetic encoding elements formed in this manner are generally known in the art as Wiegand wires.
The Wiegand wires may be used for security control cards, credit cards, identification cards and the like. Numerous wires are embedded in the card in particular positions and orientations in accordance with a desirable digital or other code scheme. Such cards can then be moved past a reading head thereby generating electrical pulses that are related to the position and orientation of the wire whereby "unique signatures" are achieved.
The Wiegand wires are formed by cyclic torsional strain and longitudinal strain to provide a bistable magnetic wire switching element having permanently different shell and core magnetic properties. The wire is circumferentially strained in alternate clockwise and counterclockwise directions while maintaining axial tension on the wire. The result is a wire which has a relatively harder magnetic shell and a relatively softer magnetic core and once magnetized the shell becomes a permanent magnet and the core, being softer, is magnetically captured by the shell to provide a return path of the lines of flux generated by the shell. When the wire is subjected to an increasing external magnetic field, a threshold is reached where the external magnetic field suddenly and rapidly captures the core to provide a low reluctance path for its flux. If the polarity of the external field is opposite from that of the shell, then the flux from the shell must be completed in the space around the wire. A pickup coil will produce a pulse in response to the sudden change in the flux pattern. Wires produced in this manner are placed in an unstrained orientation on a substrate or laminated card oriented in a first or second direction, respectively, depending upon a desired binary code to be placed on the card. These cards may then be inserted into a reader which has permanent magnets of properly oriented poles to produce the opposing magnetic flux necessary to switch or capture the cores, thereby producing pulses in adjacent pickup coils oriented to sense the sudden switch in the core magnetization direction. When these wires are removed from the reader, they revert to their original state by the outer core recapturing the center core. Wires manufactured in this manner are very difficult to produce requiring a substantial amount of equipment to properly torsion and stress the wires to produce the desired magnetic effects set out above. Further, it is difficult to properly position these individual cut lengths of wires in the proper orientation on a card or other similar device in which they are used for magnetic encoding.
In view of these problems, there is a need for a much simpler method for producing encoded elements and assembling these elements into a coded arrangement as an integral part of a card or memory device.